The present invention relates to refrigeration systems for air cooling a compartment and in particular, relates to air conditioning systems for vehicles. Systems of this type typically have refrigerant pumped by a compressor into an exothermic heat exchanger, or condenser, for liquification and expanded for flow through an endothermic heat exchanger, or evaporator, located in the compartment to be cooled and returned to the compressor in the form of gas or vapor. Vehicle passenger compartment air conditioning systems typically utilize a compressor energized by an electrical clutch connecting the compressor to the engine. In operation of modern vehicle air conditioning systems, it has been found necessary during low road speed operation of the vehicle to provide cooling fan for directing a flow of ambient air over the condenser; and, it has further been desirable to provide such a fan which is electrically energizable independently of the vehicle engine speed.
In vehicular air conditioning systems employing an electrically clutched compressor and electrically operated condenser fan, it has been desired to provide suitable sensing or warning for declutching the compressor in the event of excessive sub-cooling of the liquid refrigerant, loss of refrigerant or an overpressure condition in the high pressure liquid side of the system.
Heretofore, protection against excessive sub-cooling and overpressure or loss of refrigerant has been provided by pressure sensors or transducers disposed in the liquid refrigerant line to detect the pressure in the line. These pressure sensing devices have proven to be costly and also have exhibited a history of reliability problems. It has become increasingly necessary to provide improved control of engine ignition, fuel feed and in order to meet stringent emission requirements; and, therefore, the trend has been toward all-electrical control of engine operating parameters, including engine operated accessories, such as air conditioning. In attempting to provide all electrical control of vehicle air conditioning, and particularly malfunction alarm and shut-down, it has been difficult to combine the pressure measurements with measurements of temperature in other portions of the refrigerant system for providing integrated electrical control of the entire system.
Thus, it has been long desired to provide a way or means of providing overall control of the refrigerant system including high pressure and excessive sub-cooling warning by means of sensing only refrigerant temperature. This has proven to be difficult, inasmuch as sensing the actual temperature of the refrigerant does not permit the determination of the pressure in the refrigerant line. Thus, it has been desired to provide a way or means of electrically controlling a refrigerant system and providing warning in the event of excessive or low pressure in the refrigerant without the necessity of providing pressure sensors.